Thanks Eleshar, I'm not surprised I'm making a mistake there. I don't have a very good understanding of the concept (although I found this article to help me). Does koupim necessarily imply a future action?

Yes, it does (well there are some more complicated cases but those are largely irrelevant: narrative future, gnomic future).
There is no way, in which a verb designing single action terminated in a single point of time could exist in present.
Languages using perfective aspect (perfective, not perfect aspect!) almost never use it together with present (let it be Latin or Ancient Greek).